1. Field of the Invention
This invention relates to a hot-rolled steel sheet for use as high-strength parts such as bumper parts and impact beams of motor vehicles and, more particularly, to a high-strength hot-rolled steel sheet having excellent stretch flangeability with a tensile strength TS of not less than about 780 MPa. The invention also relates to a method of producing the hot-rolled steel sheet.
2. Description of the Related Art
In a recent trend toward lighter weight vehicle bodies, attention has been focused on application of high-strength steel sheets to a wider range of vehicle parts. In particular, high-strength steel sheets exceeding 1000 MPa have been employed as bumper parts, impact beams, etc. which are used to suppress deformation of cabins or passenger compartments upon collision of vehicles. Those high-strength steel sheets are cold-rolled steel sheets produced through a cold rolling process except for steel plate having thicknesses in excess of 3.2 mm. The main reason is that, in the case of employing cold-rolled steel sheets, disorder in shape of the steel sheet can be relatively easily suppressed by in-furnace rolls during continuous annealing and a good product shape can be obtained.
On the other hand, it has hitherto been difficult to employ hot-rolled steel sheets as thin high-strength steel sheets having thickness of not more than 3.2 mm, especially not more than 3.0 mm. One major reason is that, in a cooling step after hot rolling, effective tensile forces cannot be imparted to the steel sheet and disorder in shape of the steel sheet cannot be suppressed as with cold-rolled steel sheets.
In addition to the above-mentioned disorder in shape of the steel sheet, another reason why hot-rolled steel sheets have not been practically used as thin high-strength steel sheets having thickness not more than the above value is that the hot-rolled steel sheet is disadvantageous in ensuring satisfactory mechanical properties. More specifically, the structure just subjected to hot rolling without undergoing cold rolling and annealing is generally difficult to make uniform and achieve a fine structure comparable to that obtainable in the case of structures undergoing cold rolling and annealing. With the poor structure, it is difficult to obtain superior workability represented by stretch flangeability (bending workability and barring (Hole Expanding) workability).
To improve stretch flangeability of high-tensile hot-rolled steel sheets, several proposals have been made in the past. For example, Japanese Unexamined Patent Publication Nos. 61-19733 and 62-196336 disclose that the bainite phase is superior as a microstructure in consideration of stretch flangeability. In other words, according to those Publications, stretch flangeability is improved when a component system comprising a simple Cxe2x80x94Sixe2x80x94Mn system is subjected to accelerated cooling after hot rolling to thereby develop a structure mainly comprising bainite.
The steel sheets produced by the methods disclosed in the above-cited Japanese Unexamined Patent Publication Nos. 61-19733 and 62-196336 have excellent stretch flangeability relative to that of a steel sheet having the ferrite-martensite structure, etc., but the stretch flangeability is not sufficient to reach a level (TSxc3x97Elxe2x89xa715500 MPaxc2x7% and hole expanding ratio xe2x89xa7150%) demanded today. Further, the disclosed related art is disadvantageous in that the structure is likely to change with a comparatively high sensitivity depending on variations in the cooling start time after hot rolling and the hot rolling conditions such as the cooling rate and, therefore, the mechanical properties tend to vary to a larger extent. Such a tendency is not compatible with continuous and automatic pressing to be implemented by automobile makers and so on.
Further, Japanese Unexamined Patent Publication No. 5-320773 discloses that the effect of improving the stretch flangeability is improved by specifying the contents of S, N and O which are apt to easily produce inclusions in steel, and by adding Ti, Nb to obtain a finer structure. According to this Publication, the tensile strength of not less than 100 kgf/mm2 is satisfied by setting the coiling temperature after hot rolling to be not higher than 400xc2x0 C., and the stretch flangeability is improved by controlling the total content of (S+N+O) to be not more than 100 ppm.
With the producing method disclosed in the above-cited Japanese Unexamined Patent Publication No. 5-320773, however, the coiling temperature of not higher than 400xc2x0 C. is required to obtain the tensile strength of not less than 100 kgf/mm2 and, at such a temperature level, the mechanical properties are easily susceptible to significant variations while being in the form of a coil. Although the above-cited Japanese Unexamined Patent Publication No. 5-320773 does not clearly describe the microstructure of a hot-rolled sheet obtained by the disclosed producing method, the microstructure is presumably bainite or martensite. Then, the above disadvantage is attributable to the fact that the tensile strength can be improved, but the microstructure varies significantly and so does the tensile strength correspondingly due to the effect of variations in the steel components, the cooling conditions after hot rolling, and the temperature distribution in a coil obtained after winding the hot-rolled sheet. Such variations in the material characteristic are not compatible with continuous and automatic pressing to be implemented by automobile makers and so on.
In addition, the above-cited Japanese Unexamined Patent Publication No. 5-320773 describes the necessity of controlling the steel components to improve stretch flangeability, but the concrete relationship between the microstructure, crystal grain size, etc. and the stretch flangeability is not disclosed at all. Also, nothing is disclosed with regard to finish rolling start temperature, and coiling temperature after hot rolling is only specified to obtain the required strength.
Meanwhile, as a means for achieving the high tensile strength without performing accelerated cooling after hot rolling, there is a method of adding elements capable of improving quench hardening, such as Cu, Ni, Cr and Mo, which have been conventionally employed in the field of steel plate.
However, the method of adding the quench-hardening improving elements, such as Cu, Ni, Cr and Mo, has the problems that the necessity of using a large amount of expensive alloy elements is disadvantageous from the cost-effective point of view and renders the scrap management complicated from the viewpoint of recycling the used materials.
Further, the above known method requires the alloy elements to be added in such an amount that the added elements become perfectly a martensite single-phase. If the amount of the added alloy elements is insufficient, the resulting structure would be a mixed structure of ferrite and martensite, or a structure partly containing perlite and bainite in small amounts. Therefore, satisfactory stretch flangeability is not easy to attain as intended.
As described above, it has been very difficult to produce a high-strength hot-rolled steel sheet which has the tensile strength of not less than 780 MPa, particularly in the range of 780-1300 MPa, has good stretch flangeability, high uniformity in shape and mechanical properties of the steel sheet, and has quality enough to stand in practical use over a wide range of thickness from thickness not more than 3.0 mm corresponding to a thin steel sheet to a thickness of more than 3.0 mm corresponding to a thick steel sheet that is produced as an ordinary hot-rolled steel sheet. Accordingly, there has been a strong demand for development of the technique for producing a hot-rolled steel sheet, which can succeed in overcoming the problems set forth above. From the viewpoint of reducing the cost of steel sheets, in particular, there has been demanded a technique of producing a hot-rolled steel sheet with a composition of low-alloy system containing alloy elements in amount as small as possible.
With the view of overcoming the above-mentioned problems encountered in the related art, an object of the present invention is to provide a thin high-strength hot-rolled steel sheet which has excellent stretch flangeability and high uniformity in both shape and mechanical properties of the steel sheet, and to provide a method of producing the hot-rolled steel sheet.
Another object of the present invention is to provide an inexpensive producing technique which can produce the high-strength hot-rolled steel sheet even with a thickness of not more than 3.5 mm and a composition of low-alloy system.
Still another object of the present invention is to provide the high-strength hot-rolled steel sheet having the tensile strength of not less than 780 MPa as a target value for one practical characteristic of the steel sheet.
To achieve the above objects, the inventors conducted intensive experiments and studies from the standpoints of steel components, producing conditions, etc.
As a result, the inventors discovered that, by producing hot-rolled steel sheets under combination of steel having a composition adjusted to a proper component range and proper hot rollingxe2x80x94cooling conditions, a uniform and fine structure mainly comprising bainite can be formed and good mechanical properties can be obtained with stability without using expensive alloy elements.
It was also found that, of the producing conditions, control of a cooling pattern after the hot rolling and the coiling temperature after the hot rolling are important to obtain a uniform and fine bainite structure. More specifically, in conventional cooling on a hot run table, attention has been focused only on an average cooling rate from the start of the cooling to the coiling, and no consideration has been paid to cooling rates at respective positions on the hot run table. Further, in steel having the composition according to the present invention, the xcex3-structure is transformed into a desired microstructure at the time of coiling after the cooling, whereby the steel is provided with required mechanical characteristics such as tensile strength. However, it has been conventional to control only an average temperature over the entire length of a hot-rolled sheet coil having a width of 70 cm-120 cm and a length of 300 m-900 m, or to control only the temperature of the coil in its outer peripheral portion. Thus, the temperature of the hot-rolled sheet under coiling in the transverse direction and the temperature of the inside of the coil have not been controlled.
With those conventional methods, therefore, the shape and mechanical characteristics of the steel sheet are varied significantly due to variations in microstructure of the coiled steel sheet in the transverse and longitudinal directions, and the steel sheet having uniform mechanical properties enough to stand in practical use has not been obtained.
The inventors found that, to overcome the above-mentioned problem, it is very effective to continuously cool the hot-rolled steel sheet on the hot run table without interruption while holding a predetermined cooling rate (comparatively slow cooling) during cooling until the start of coiling after hot rolling, and to control the coiling temperature to fall in a proper range. Then, the inventors reached the conclusion that the above objects can be achieved by combining a proper steel composition with proper hot rolling conditions (such as a slab heating temperature and a finish rolling start temperature).
The present invention has been accomplished on the basis of the above findings and has the following features.
(1) In a high-strength hot-rolled steel sheet having excellent stretch flangeability, the steel sheet has a composition containing:
C: about 0.05-0.30 wt %,
Si: about 0.03-1.0 wt %,
Mn: about 1.5-3.5 wt %,
P: not more than about 0.02 wt %,
S: not more than about 0.005 wt %,
Al: not more than about 0.150 wt %,
N: not more than about 0.0200 wt %,
one or two of Nb: about 0.003-0.20 wt % and Ti: about 0.005-0.20 wt %,
B: about 0.0005-0.0040 wt % as an optionally added element,
one or more of Cu: about 0.02-1.0 wt %, Ni: about 0.02-1.0 wt %, Cr: about 0.02-1.0 wt %, and Mo: about 0.02-1.0 wt %, as an optionally added elements, in total content of not more than about 1.0 wt %,
Ca: about 0.0005-0.0050 wt % as an optionally added element, and
the balance consisting of Fe and inevitable impurities,
the steel sheet having a microstructure that contains fine bainite grains with a mean grain size of not greater than about 3.0 xcexcm at an area percentage of not less than about 90%.
(2) In the high-strength hot-rolled steel sheet having excellent stretch flangeability as recited in paragraph (1), an aspect ratio of the fine bainite grains is not more than about 1.5.
(3) In the high-strength hot-rolled steel sheet having excellent stretch flangeability as recited in any of paragraphs (1) and (2), a maximum size of the major axis (usually in the rolling direction) of the fine bainite grains is not greater than about 10 xcexcm.
(4) In a method of producing a high-strength hot-rolled steel sheet having excellent stretch flangeability, the method comprises the steps of preparing a slab containing C: about 0.05-0.30 wt %, Si: about 0.03-1.0 wt %, Mn: about 1.5-3.5 wt %, P: not more than about 0.02 wt %, S: not more than about 0.005 wt %, Al: not more than about 0.150 wt %, N: not more than about 0.0200 wt %, and one or two of Nb: about 0.003-0.20 wt % and Ti: about 0.005-0.20 wt %; heating the slab at a temperature of not higher than about 1200xc2x0 C.; hot rolling the slab at a finish rolling end temperature of not lower than about 800xc2x0 C., preferably at a finish rolling start temperature of about 950-1050xc2x0 C.; starting to cool a hot-rolled sheet within about two seconds after the end of the rolling step; continuously cooling the hot-rolled sheet down to a coiling temperature at a cooling rate of about 20-150xc2x0 C./sec; and coiling the hot-rolled sheet at a temperature of about 300-550xc2x0 C., preferably in excess of 400xc2x0 C.
Details of the present invention will be apparent from the Description of the Preferred Embodiments, Brief Description of the Drawings, and Examples given below.
Additionally, it is to be noted that the invention is not limited by Description of the Preferred Embodiments, Brief Description of the Drawings, and Examples given below.